The present invention relates to a print head having an array of print elements each driven to print a dot at a time. More particularly, the invention relates to adjustment of the driving power with which each of the print elements is driven. A typical example of such a printer is an electrophotographic printer having an array of LEDs (light-emitting diodes) as the print elements, and the driving power is in the form of a current supplied to each LED.
Electrophotographic printers have been widely used as printers for printing texts and pictures prepared by computers or the like. FIG. 8 shows the configuration of a printing unit in such an electrophotographic printer. When texts or pictures are printed, negative charges are applied by a charging roller 1 to a rotating photosensitive drum 2 to negatively and uniformly charge the photosensitive drum 2. A print head 3 comprising a plurality of LEDs illuminate the photosensitive drum 2 to form an electrostatic latent image of the text or picture to be printed. A developer 5 causes negatively-charged toner 6 to be adhered to the electrostatic latent image to achieve development. A transfer roller 4 to which a positive voltage is applied from a transfer power supply 8 transfers the toner on the photosensitive drum 2 to a printing paper 7. The toner is thereafter fixed on the paper 7 by a fixing device, not shown.
The print head 3 forming the electrostatic latent image in this way is formed of an LED array, an LED driver and the like. The LED array comprises a plurality of LED array units each of which is in the form of a separate semiconductor chip, and the entire LED array is formed of several thousands LEDs aligned in the direction perpendicular to the direction of the paper transport in FIG. 8. The LED driver is formed of as many drive transistors as there are LEDs. The drive transistors are selectively turned on depending on the image data representing the text or picture to be printed. The period for which the transistors are turned on is determined by a common strobe signal supplied from a control unit controlling the entire printer. When the transistors are turned on, a current is flows to the LEDs connected with the transistors.
The LEDs in the LED array in the print head have certain variations in the relationship between the current supplied thereto, and the amount of light emitted therefrom, due to fluctuations or the like in the manufacturing conditions in the factory. The amount of light emitted with respect to the current supplied thereto is called light emitting efficiency. Such variations are more prominent between LED array units than between individual LEDs in each LED array unit.
There is also variations in the characteristics (due to the differences in the resistance of the semiconductor elements) between drive transistors in the LED driver. Some transistors permit a large current to flow while others permit a small current, under the same voltage. Such a variation is also more prominent between LED driver ICs than between the individual transistor elements in each LED driver IC. Because of the variations in the characteristics of the LEDs and the transistors, LED array units and LED driver ICs are classified into several ranks according to their characteristics, during the manufacture of the LED array units and the LED driver ICs, and LED array units and LED driver ICs of the same rank are used together to form a head. By such classification, one LED head is formed of LED array units having a high light emitting efficiency, while another LED head is formed of LED array units of a relatively low light emitting efficiency. The variation in the characteristics between the LED array units in each LED head can be reduced in this way. Similarly, one LED head is formed of driver ICs having transistors of a high resistance, and another LED head is formed of driver ICs having a low resistance.
With the prior art print heads, to reduce the difference in the amount of emitted light between LED elements, a non-volatile storage element is provided in the LED head, and correction (or compensation) data are set for use in correcting the current flowing through the LEDs. During the manufacture of the head, the correction data are determined in the following manner. First, provisional correction data are set, and written in the non-volatile storage circuit in the LED head, and transferred to the LED driver, and the amount of light emitted from each LED driven in accordance with the provisional correction data is measured, and second provisional correction data are set by modifying the first provisional correction data on the basis of the result of the measurement. Such a procedure is repeated, such that appropriate correction data is finally obtained.
The optimum amount of energy (total amount of emitted light) per dot used for exposure of the photosensitive drum is predetermined. The total amount of emitted light can be represented by the product of the light emission power and the length of time period of the emission, i.e., the time period for current flow (i.e., strobe time). Accordingly, the strobe time must be determined to an appropriate value based on the characteristics of the LEDs and transistors. For instance, a relatively short strobe time must be set for an LED head formed of LEDs having a high light emitting efficiency, while a relatively long strobe time must be set for an LED head formed of LEDs having a low light emitting efficiency.
In order to set the strobe time in this way, during the manufacture of the LED heads, the average light emitting power of the dots of the LED head is determined, and is written in a bar code label, which is then attached to the print head. During assembly of the printer, i.e., during assembly of the printing unit and control unit to complete a printer, the average light emitting power written in the bar code label is read, and used for calculating the strobe time, and the strobe time data representing the calculated strobe time is written in the non-volatile memory in the control unit. During use of the printer for printing, the control unit sends a strobe signal of a time length represented by the strobe time data read from the non-volatile memory, to the LED head, together with image data to be printed, received from a computer. In this way, the LEDs emit light only for the duration of the strobe time, in forming the electrostatic latent image on the photosensitive drum.
In adjusting or optimizing the correction data for the conventional print head, writing and erasure of the non-volatile memory, whose operating speed is low, are repeated many times, so that a long time is needed, and as a result, the time required for the manufacture of the printer is long, with the result that the cost of manufacture of the printer becomes high.
Moreover, attaching a bar code label, and reading the information from the bar code label also lengthens the time for manufacturing the printer, and the cost of manufacture of the printer is further increased.
An object of the invention is to solve the problems described above.
Another object of the invention is to provide a print head which can be manufactured in a shorter period, and at a lower cost.
According to a first aspect of the invention, there is provided a print head of a printer comprising an array of print elements, including:
a plurality of print elements each selectively driven to print a dot;
a driver formed of a plurality of drive elements provided for the respective print elements, for driving the print elements in accordance with print data representing the image to be printed and correction data for adjusting the driving power for each print element;
a storage circuit into which the correction data input from the outside of the print head are written, and from which the correction data are sent to the driver; and
a sending means for sending the correction data supplied from the outside to the driver.
According to a second aspect of the invention, there is provided a print head of a printer comprising an array of print elements, including:
a plurality of print elements each selectively driven to print a dot;
a driver formed of a plurality of drive elements provided for the respective print elements, for driving the print elements in accordance with print data representing the image to be printed and correction data for adjusting the driving power for each print element;
a storage circuit into which the correction data input from the outside of the print head are written, and from which the correction data are sent to the driver; and
wherein the correction data stored in the storage circuit is read and sent to the outside of the print head via said sending means.